Cleopatra eBook

The long series of patient, careful, and sagacious
observations, which have been continued now for two
thousand years, bring us results, by means of which,
through our powers of mental conception, we may take
a comprehensive survey of the whole scene, analogous,
in some respects, to that which direct and actual
vision would afford us, if we could look down upon
it from the eagle’s point of view. It is,
however, somewhat humiliating to our pride of intellect
to reflect that long-continued philosophical investigations
and learned scientific research are, in such a case
as this, after all, in some sense, only a sort of substitute
for wings. A human mind connected with a pair
of eagle’s wings would have solved the mystery
of Egypt in a week; whereas science, philosophy, and
research, confined to the surface of the ground, have
been occupied for twenty centuries in accomplishing
the undertaking.

It is found at last that both the existence of Egypt
itself, and its strange insulation in the midst of
boundless tracts of dry and barren sand, depend upon
certain remarkable results of the general laws of
rain. The water which is taken up by the atmosphere
from the surface of the sea and of the land by evaporation,
falls again, under certain circumstances, in showers
of rain, the frequency and copiousness of which vary
very much in different portions of the earth.
As a general principle, rains are much more frequent
and abundant near the equator than in temperate climes,
and they grow less and less so as we approach the
poles. This might naturally have been expected;
for, under the burning sun of the equator, the evaporation
of water must necessarily go on with immensely greater
rapidity than in the colder zones, and all the water
which is taken up must, of course, again come down.

It is not, however, wholly by the latitude of the
region in which the evaporation takes place that the
quantity of rain which falls from the atmosphere is
determined; for the condition on which the falling
back, in rain, of the water which has been taken up
by evaporation mainly depends, is the cooling of the
atmospheric stratum which contains it; and this effect
is produced in very various ways, and many different
causes operate to modify it. Sometimes the stratum
is cooled by being wafted over ranges of mountains,
sometimes by encountering and becoming mingled with
cooler currents of air; and sometimes, again, by being
driven in winds toward a higher, and, consequently,
cooler latitude. If, on the other hand, air moves
from cold mountains toward warm and sunny plains,
or from higher latitudes to lower, or if, among the
various currents into which it falls, it becomes mixed
with air warmer than itself, its capacity for containing
vapor in solution is increased, and, consequently,
instead of releasing its hold upon the waters which
it has already in possession, it becomes thirsty for
more. It moves over a country, under these circumstances,
as a warm and drying wind. Under a reverse of
circumstances it would have formed drifting mists,
or, perhaps, even copious showers of rain.